1. Field of the Invention
The present invention is directed to the field of control for electric motors and, more specifically, to the area of motor protection by detecting excessive current due to stalling in such motors.
2. Description of the Prior Art
Various solutions have been proposed in the past for limiting the effects of excessive currents in electric motors and associated control circuitry. In the automotive environment, such motors are usually energized with DC voltage to provide limited range movement of windows or seats. Stalling may occur when the mechanism driven by the motor reaches the end of its travel path or is otherwise prevented from moving. At such a point, a motor will draw excessive current that can cause damage to not only the motor but the switching circuitry that supplies current to the motor.
In commonly assigned U.S. Pat. No. 4,725,765 (incorporated herein by reference) a circuit is described for detecting the stall current in a DC motor and inhibiting the application of DC voltage to the motor for a predetermined period of time. In the '765 patent, it is recognized that the current drawn by the motor immediately, upon start-up, peaks at a high level with respect to the current drawn by the motor during its subsequent run condition. At stall, the power supplied to the motor is inhibited for predetermined periods of time to reduce the effects of the stall current on the motor. The system described in the '765 patent looks for a stall current level that is continuous for a period of time greater than that which occurs during the start-up period, in order to distinguish the two.
In most prior art attempts to address the problem of detecting stall currents, certain assumptions are made as to the magnitude of the stall current level. However, it has been found that the current draw characteristics of motors and associated linkages tend to vary one to another and over time. In addition, motors are temperature sensitive and tend to draw more current at low temperatures.